(1) Field of the Invention:
The present invention relates to oxygen sensors which can always provide optimum measurements without being influenced by a direction of contact of the measurement gas.
(2) Related Art Statement:
In order to improve the measuring accuracy of an oxygen sensor using a planar sensor element, Japanese Utility Model Registration Application Laid-open No. 60-150,447, NGK Insulators, Ltd. disclosed the oxygen sensor shown in FIG. 5a, in which a protection cover 22 protecting a planar sensor element 21 is provided with flow holes 23, through which a gas to be measured is introduced, which are not located at a position opposed to a wider surface of the sensor element. Reference numeral 25 denotes a gas discharge opening.
However, in the oxygen sensor described in Japanese Utility Model Registration Application Laid-open No. 60-150,447, a detecting section 24 is provided on the wider surface of the planar sensor element 21 as shown in FIG. 5b (a Vb--Vb sectional view of FIG. 5a). Therefore, the oxygen sensor has a shortcoming in that a direction in which the gas to be measured contacts the detecting section 24 depending upon the fitting direction thereof. The way in which the gas contacts the detecting section 24 differs between a case where the gas enters in an A-direction and a case where the gas enters in a B-direction. If the gas to be measured contacts the detecting section 24 in different directions like this, a .lambda. control point of the sensor varies for reasons mentioned later. Consequently, high accuracy measurements cannot be obtained.